Hi,
Yes, this is a caching issue, and the portability problem you have discovered arises because persistence provider implementations use different default caching strategies.
JPA 2.0 provides support to address this at different levels of granularity. Have a look at the persistence.xml shared-cache-mode element, the Cache interface, and the cache retrieve mode and cache store mode properties that can be passed to entity manager and query operations.
-Linda
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